Bearings are devices that permit constrained relative motion between two parts. They may be used in many different types of machinery to retain and support rotating components such as, for example, a wheel on a vehicle, a vane on a windmill or a drum in a washing machine. A typical bearing comprises inner and outer rings and a plurality of rolling elements, and may further comprise a cage to retain the rolling elements. During use, the bearing is subjected to different loads, both static and dynamic. The static load is mainly due to the weight supported by the bearing and may also be due to a preload with which the bearing is mounted. The dynamic loads are time-dependent and are due to the operating conditions.
In many systems, it is desirable to be able to monitor the load acting on a bearing. In modern vehicles, for example, load data from the wheel bearings are used in the control of vehicle stability systems. Displacement sensors such as strain sensors are commonly applied to bearings to measure load and deformation. In some cases, plastic foil sensors are adhesively bonded to e.g. an outer circumference of a bearing outer ring. Adhesive bonding has many disadvantages, however. The bearing surface must be cleaned and prepared for bonding, the attachment is a manual process, the adhesive can take several hours to cure and, moreover, the adhesive can be subject to creep over time, which impairs the reliability of the sensor signal.
Metal foil strain sensors can also be used. Such strain sensors are attached to a bearing surface by means of e.g. spot welding, whereby the metal foil is attached around its entire periphery using a specific pattern of spot welds. The attachment of foil strain sensors to a bearing surface is again an essentially a manual process, making these sensors unsuited for automated mounting and integration in a production-line environment. Furthermore, foil strain sensors have a low strain sensitivity, which places stringent requirements on a signal conditioner when used at the low operating voltages common in contemporary electronic systems.
In JP 2007239848, a bearing provided with a sensor unit is suggested, whereby the sensor unit comprises at least one sensor mounting member and at least one strain sensor mounted on the sensor mounting member. The sensor mounting member is attached to a radially outer surface of the bearing outer ring at two fixing portions separated from each other in the circumferential direction, and the strain sensor is disposed on the sensor mounting member between the two fixing portions. The sensor mounting member may be attached to the bearing outer ring by means of a bolted connection, adhesive bonding or welding.
There is room for improvement, however, in terms of realizing a strain sensor that can be attached to a bearing surface, or other component surface, in a straightforward automated process and which, after attachment, enables a strong and stable strain signal to be sensed.